Hi,
you can do something like this:
1. Create 2 fields
- Link field for link URL (hidden on profile page) - for example with alias "reservation_link"
- HTML delimiter (hidden on registration and edit profile pages) - for example with alias "reservation_btn"
2. Put in HTML delimiter put something like this:

<a class="btn btn-default" href="/{user reservation_link displayed raw}">Reservation</a>

3. In the link field add condition to hide HTML delimiter if it is empty, so:
Condition 1
Operator: "Equal"
Field to Compare: "Custom Value"
Custom Value: Leave empty
Target Field: Choose HTML Delimiter
Target Action: Hide

Links
{user reservation_link displayed raw} will be replaced with URL in raw mode.
This sintax become from our Content Plugin, see docs at http://docs.easy-profile.com/index.php/article/content-plugin
Docs about HTML delimiter field type at http://docs.easy-profile.com/index.php/article/html-delimiter-field-type

Tips
- You can use a simple Custom module to show button, you need to enable "Prepare Content" parameter to trigger Contents Plugin, HTML delimiter is useful to show button in Field Position.
- a Good Alternative is to use a simple javascript to add class and change text of link.